1. Field of the Invention
The present invention relates to an identification tag (ID-tag) for wirelessly communicating with a remote controller, to which electric power is wirelessly supplied from the remote controller. The remote controller is, for example, a portable reader-writer.
2. Description of Related Art
A radio communication system using high frequency wave between a remote controller and an ID-tag carried by a moving body has been known hitherto. In this system, data stored in the ID-tag are read and data are written on the ID-tag through wireless communication between the ID-tag and the remote controller. The ID-tag includes a modulator-demodulator for transmitting and receiving data, a memory and a microprocessor unit for controlling communication. Usually, the ID-tag also includes a power source such as a battery. If a battery is used as a power source, life of the battery determines the life of the ID-tag, i.e., the ID-tag cannot be used anymore when the battery power is used up.
To solve the problem of battery life, a system in which power is wirelessly supplied from an outside source such as a remote controller has been in use recently. In this system, the ID-tag includes a resonant circuit consisting of an antenna coil and a condenser. Power is received by the resonant circuit as alternating current power, and then it is rectified by a rectifier included in the ID-tag. This kind of ID-tag is attached to an article such as a pallet carrying parts on a conveyer of a manufacturing line or a package sequentially transported in a depot. Since these articles are transported sequentially one by one, electric power to operate the ID-tag can be effectively supplied to the ID-tag form the remote controller. However, if the articles each having the ID-tag are stacked together, electric power cannot be supplied to all the ID-tags, because ID-tags located close to an ID-tag to which electric power is being supplied cannot receive electric power. This is because electric current flowing in the resonant circuit in the ID-tag which receives electric power from the remote controller functions to cancel radio wave which is to be supplied to other ID-tags located nearby.
In addition, when a portable reader-writer such as a hand-held terminal is used as the remote controller, it is necessary to keep the portable reader-writer at a same position until communication with all the stacked articles is completed.